Overview of Wireless Communications Networks
In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as mobile stations or user equipments, communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-comprised, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “eNodeB” or “NodeB” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment installed at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, .e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks. In some networks, there is also an interface between radio nodes, e.g., the X2 interface between eNodeBs in LTE.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Long Term Evolution (LTE) together with Evolved Packet Core (EPC) is the newest addition to the 3GPP family.
Radio measurements play a key role in wireless communications. At a general level, radio measurements may be categorized into signal strength/quality measurements, timing measurements, and other measurements. The measurements may be performed by the user equipment and/or radio network nodes equipped with a radio interface. The different categories or radio measurements, and other network aspects related to radio measurements, are described in greater detail below according to the provided sub-headings.
Signal Strength and Quality Measurements
Examples of LTE measurements characterizing signal strength or quality of a given cell are Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), received interference power, and thermal noise power. RSRP and RSRQ are currently defined as user equipment measurements, e.g., in DL, and associated with cell-specific reference signals (CRS). However, received signal strength and received signal quality measurements are known to be more general, e.g., for any type of signal and for DL and UL. Similar measurements exist in UMTS, GSM and CDMA2000, etc.
Timing Measurements
In LTE, the following user equipment timing measurements have been standardized since release 9, user equipment Rx−Tx time difference, Reference Signal Time Difference (RSTD), and user equipment GNSS Timing of Cell Frames for user equipment positioning. The following E-UTRAN measurements have been standardized since release 9, eNodeB Rx−Tx time difference, Timing Advance (TA),TA Type 1=(eNB Rx−Tx time difference)+(user equipment Rx−Tx time difference),TA Type 2=(eNB Rx−Tx time difference), and E-UTRAN GNSS Timing of Cell Frames for user equipment positioning.
In addition, there may also be measurements that are not explicitly standardized, but may still be implemented by user equipment or E-UTRAN or standardized later. Some examples of these measurements may be time of arrival, measured by radio node, e.g., eNodeB or a radio measurement node such as LMU,RSTD measured by radio nodes, one way propagation delay, measured by eNode B for estimation of timing advanced to be signaled to the user equipment (a similar user equipment measurement may be defined in the future), and timing measurements over multifarious links. Similar measurements may also exist in other RATs, e.g., Rx−Tx measurements may be similar to Round Trip Time (RTT) measurements in UMTS, and RSTD may be similar to a System Frame Number (SFN)-to-SFN time difference in UMTS.
Timing measurements may be used for positioning (e.g., with Enhanced Cell Identification (E-CID), Adaptive Enhanced Cell ID (AECID), pattern matching, Observed Time Difference of Arrival (OTDOA), Uplink Time Difference of Arrival (U-TDOA), hybrid positioning methods), Minimization of Drive Tests (MDT), network planning, Self-Optimizing/Organizing Network (SON), enhanced inter-cell resource and interference coordination (eICIC) and heterogeneous network (HetNet) (e.g., for optimizing the cell ranges of different cell types), configuration of handover parameters, time-coordinated scheduling, etc. Measurements of a general purpose are typically configured by the serving/primary cell. Measurements of a specific purpose may be configured by other nodes, e.g., by positioning node (e.g., Evolved Serving Mobile Location Centre (E-SMLC) or Secure User Plane Location Platform (SLP) in LTE), SON node, MDT node, etc.
Timing advance may also be used to control the timing adjustment of user equipment UL transmissions. The adjustment is transmitted to the UE in the timing advance command. In LTE, for user equipments not supporting LPP, the user equipment timing adjustment may be based on TA Type 2.
User equipment measurements configured by the network are typically reported to a network node, e.g., eNodeB, positioning node, etc. Radio node measurements may also be reported to a network node, e.g., another radio node such as eNodeB or LMU, or other network node such as positioning node. Some measurements may be not reported but used internally by the measuring node, including the user equipment. Furthermore, some measurements may involve both directions (DL and UL), e.g., Rx−Tx measurements. It should also be appreciated that the user equipment may also be involved in the radio node (e.g., eNodeB) measurements such as Rx−Tx measurements, and eNodeB may also be involved in the user equipment measurements such as Rx−Tx measurement.
Other Measurements
An example of a measurement that does not belong to the first two groups of measurements is an Angle of Arrival (AoA) measurement. In the current LTE standard, AoA is defined as an E-UTRAN measurement. However, AoA measurements performed by the user equipment are also known.
Inter-frequency, Inter-band, and Inter-RAT Measurements
User equipments typically support all intra-RAT measurements (i.e. inter-frequency and intra-band measurements) and meet the associated requirements. However the inter-band and inter-RAT measurements are user equipment capabilities, which are reported to the network during the call setup. The user equipment supporting certain inter-RAT measurements should meet the corresponding requirements. For example a user equipment supporting LTE and WCDMA should support intra-LTE measurements, intra-WCDMA measurements and inter-RAT measurements (i.e. measuring WCDMA when serving cell is LTE and measuring LTE when serving cell is WCDMA). Hence, the network can use these capabilities according to its strategy. These capabilities are highly driven by factors such as market demand, cost, typical network deployment scenarios, frequency allocation, etc.
Inter-frequency Measurements
Inter-frequency measurements involve measurements on at least one cell (e.g., RSTD measurement involves two cells) that belong to a frequency/carrier different from the serving/primary cell frequency/carrier. Examples of inter-frequency measurements are inter-frequency RSRP, inter-frequency RSRQ, inter-frequency RSTD, etc.
The user equipment performs inter-frequency and inter-RAT measurements in measurement gaps. The measurements may be done for various purposes: mobility, positioning, self organizing network (SON), minimization of drive tests, etc. Furthermore, the same gap pattern is used for all types of inter-frequency and inter-RAT measurements. Therefore E-UTRAN must provide a single measurement gap pattern with constant gap duration for concurrent monitoring (i.e. cell detection and measurements) of all frequency layers and RATs.
Inter-RAT Measurements
In general, in LTE inter-RAT measurements are typically defined similar to inter-frequency measurements, e.g. they may also require configuring measurement gaps like for inter-frequency measurements, but just with more measurements restrictions and often more relaxed requirements for inter-RAT measurements. As a special example, there may also be multiple networks using the overlapping sets of RATs. The examples of inter-RAT measurements specified currently for LTE are UTRA FDD CPICH RSCP, UTRA FDD carrier RSSI, UTRA FDD CPICH Ec/No, GSM carrier RSSI, and CDMA2000 1× RTT Pilot Strength. LTE FDD and TDD may also be treated as different RATs.
Inter-band Measurements
An inter-band measurement refers to the measurement done by the user equipment on a target cell on the carrier frequency belonging to the frequency band which is different than that of the serving/primary cell. Both inter-frequency and inter-RAT measurements can be intra-band or inter-band.
The motivation for inter-band measurements is that most of the user equipments today support multiple bands even for the same technology. This is driven by the interest from service providers; a single service provider may own carriers in different bands and would like to make efficient use of carriers by performing load balancing on different carriers. A well-known example is that of multi-band GSM terminal with 800/900/1800/1900 bands.
Furthermore, a user equipment may also support multiple technologies e.g. GSM, UTRA FDD and E-UTRAN FDD. Since all UTRA and E-UTRA bands are common, therefore the multi-RAT user equipment may support same bands for all the supported RATs.
Carrier Aggregation (CA) Networks
A multi-carrier system (or interchangeably called as the carrier aggregation (CA)) allows the user equipment to simultaneously receive and/or transmit data over more than one carrier frequency. Each carrier frequency is often referred to as a component carrier (CC) or simply a serving cell in the serving sector, more specifically a primary serving cell or secondary serving cell. The multi-carrier concept is used in both HSPA and LTE. Carrier aggregation is supported for both contiguous and non-contiguous component carriers, and component carriers originating from the same eNodeB need not to provide the same coverage. Furthermore, carriers may also belong to different RATs. Below definitions are provided for various cells in a CA network.
Serving Cell: for a user equipment in RRC_CONNECTED not configured with CA there may be only one serving cell comprising the primary cell. For a user equipment in RRC_CONNECTED configured with CA, the term ‘serving cells’ is used to denote the set of one or more cells comprising of the primary cell and all secondary cells.
Primary Cell (PCell): the cell, operating on the primary frequency, in which the user equipment either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or the cell indicated as the primary cell in the handover procedure.
Secondary Cell (SCell): a cell, operating on a secondary frequency, which may be configured once an RRC connection is established and which may be used to provide additional radio resources.
In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). Depending on user equipment capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to a SCell is a Downlink Secondary Component Carrier (DL SCC) while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).
In CA the base station (e.g. eNode B) in LTE can deactivate one or more secondary cells on the corresponding secondary carriers. The deactivation is done by the eNode Busing lower layer signaling (e.g. over PDCCH in LTE) using a short command such as ON/OFF (e.g. using 1 bit for each SCell). The activation/deactivation command is sent to the user equipment via the PCell. Typically the deactivation is done when there is no data to transmit on the SCell(s). The activation/deactivation can be done independently on uplink and downlink SCell. The purpose of the deactivation is thus to enable user equipment battery saving. The deactivated SCell(s) can be activated also by the same lower layer signaling.
Cell Change in LTE
Herein, a cell change is referred to as changing the cell to which the user equipment is associated to. The cell change may further refer, for example, to:                serving cell change (e.g., at handover in a non-CA system or when the user equipment is not configured with any SCell),        serving cell set change (e.g., in a CA system adding/removing/modifying an SCell),        PCell change (e.g., in a CA system changing the current PCell being cell with the first cell identity to another cell with the second cell identity).        
A cell change may occur, for example, during:                Handover (intra-frequency, inter-frequency or inter-RAT), or        PCell change on the same PCC (in a CA system), or        Carrier switching (changing the current PCC to another frequency carrier, which implies also PCell change).        
A cell change may be due to e.g. mobility, load balancing, energy saving, carrier activation/deactivation or cell activation/deactivation, secondary carrier activation/deactivation or secondary cell (or secondary serving cell) activation/deactivation, etc.
Measurement Requirements at a Cell Change
Most of the measurements characterize a signal of a specific cell, e.g., a serving or a neighbor cell. Some of the measurements relate to signals of two specific cells, e.g., relative measurements such as RSTD between a neighbor and a reference cell. A few measurements characterize the radio environment at a specific location (e.g., interference- and noise-related measurements such as Thermal noise power, Received Interference Power, RSSI or Noise Rise).
A measurement may be specified for a certain cell (e.g., identified by the cell identity) or a certain cell category (e.g., a serving cell, reference cell, neighbor cell). The cell identification of the same cell does not change when e.g. the serving cell change occurs for a user equipment. However, the category of a cell may or may not change when the user equipment is moving from one cell to another cell, e.g., the serving cell changes during handover or carrier switching, but OTDOA reference cell may not change. Therefore, the measurements associated with a certain cell (e.g., like in OTDOA) may in principle continue after, e.g., handover, whilst the measurement associated with a certain cell category may need to be stopped or restarted at handover, depending on the measurement and cell category.
Example 1: Requirements for User Equipment Rx−Tx Measurements for Positioning When Handover Occurs
The current standard specifies that if the user equipment is performing user equipment Rx−Tx time difference measurement while the serving cell is changed due to the handover then the user equipment shall restart the Rx−Tx measurement on the new cell. In this case the user equipment shall also meet the user equipment Rx−Tx time difference measurement and accuracy requirements. However the physical layer measurement period of the user equipment Rx−Tx measurement shall not exceed Tmeasure_FDD_UE_Rx_Tx3 as defined in the following expression:
Tmeasure_FDD_UE_Rx_Tx3=(K+1)*(Tmeasure_FDD_UE_Rx_Tx1)+K*TPCell_change_handover, where K is the number of times the serving cell is changed over the measurement period (Tmeasure_FDD_UE_Rx_Tx3), TPCell_change_handoveris the time to change the serving cell due to handover; it can be up to 45 ms.
Example 2: Requirements for User Equipment Rx−Tx Measurements for Positioning When PCell Switching Occurs with Carrier Aggregation.
If the user equipment supporting E-UTRA carrier aggregation when configured with the secondary component carrier is performing user equipment Rx−Tx time difference measurement while the PCell is changed regardless whether the primary component carrier is changed or not then the user equipment shall restart the Rx−Tx measurement on the new PCell. In this case the user equipment shall also meet the user equipment Rx−Tx time difference measurement and accuracy requirements. However the physical layer measurement period of the user equipment Rx−Tx measurement shall not exceed Tmeasure_FDD_UE_Rx_Tx2 as defined in the following expression: Tmeasure_FDD_UE_Rx_Tx2 =(N+1)*(Tmeasure_FDD_UE_Rx_Tx1)+N*TPcell_change_CA, where: N is the number of times the PCell is changed over the measurement period (Tmeasure_FDD_UE_Rx_Tx2), TPcell_change_CA is the time to change the PCell; it can be up to 25 ms.
For OTDOA, the user equipment performs RSTD measurements with respect to the reference cell, so in general the user equipment should be able to continue the RSTD measurements after the serving/primary cell changes when the assistance data is provided with respect to a reference cell which is not restricted to be the serving cell.
Impact of RF Receiver Reconfiguration on Measurement
In single carrier LTE, the cell may operate at the channel bandwidths ranging from 1.4 MHz to 20 MHz. However, single-carrier legacy user equipment shall be able to receive and transmit over 20 MHz, i.e., the maximum single-carrier LTE bandwidth. If the serving cell bandwidth is smaller than 20 MHz, then the user equipment may also shorten the bandwidth of its RF front end. For example, if the serving cell bandwidth (BW) is 5 MHz, then the user equipment may also configure its RF BW to 5 MHz. This approach has several advantages. For example it enables the user equipment:                To prevent the user equipment from the noise outside the current reception bandwidth,        To save its battery life by lowering the power consumption.        
The reconfiguration of the user equipment reception and/or transmission bandwidth involves some delay, e.g., 0.5-2 ms or longer, depending upon user equipment implementation and also whether both UL BW and DL BW are reconfigured at the same time or not. This small delay is often referred to as a ‘glitch’. During the glitch the user equipment cannot receive from the serving cell or transmit to the serving cell. Hence this may lead to interruption in data reception/transmission from/to serving cell. The user equipment is also unable to perform any type of measurements during the glitch. The glitch occurs either when the user equipment extends its bandwidth (e.g. from 5 MHz to 10 MHz) or when it shortens its bandwidth (e.g. from 10 MHz to 5 MHz).
Furthermore, when the user equipment operates at a bandwidth lower than its maximum reception capability and the user equipment wants to measure over a larger than the current bandwidth, e.g., for measuring a cell on the same frequency, then it has to open its receiver for performing the measurement. Thus, in this case (i.e. when current BW<max BW) the glitch occurs before and after the user equipment obtains each measurement sample, if the user equipment reconfigures back to its current operation after each measurement sample over the larger bandwidth. On the other hand, keeping the receiver open, e.g. up to max bandwidth, to enable a measurement of a larger-bandwidth neighbor cell on the same frequency without a glitch when the system bandwidth of a first measured cell is smaller than max BW would lead to performance degradation of the first cell measurements.
The glitch also occurs when the CA capable user equipment reconfigures its bandwidth from single carrier to multiple carrier mode or vice versa, or when activating/deactivating CA cells or component carriers. For example, consider CA capable user equipment supporting 2 DL component carriers each of 20 MHz: PCC and 1 SCC. If the secondary component carrier is deactivated by the serving/primary cell then the user equipment will shorten its BW e.g. from 40 MHz to 20 MHz. This may cause 1-2 ms or even longer interruption on the PCC.
Positioning Architecture in LTE
The three key network elements in an LTE positioning architecture are the LCS Client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device (typically a user equipment or a radio node) by collecting measurements and other location information, assisting the terminal in measurements when necessary, and estimating the LCS target location. A LCS Client is a software and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e. the entities being positioned. LCS Clients may reside in a network node, radio network node, a user equipment, and it may also reside in the LCS targets themselves. An LCS Client sends a request to LCS Server to obtain location information, and LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request can be originated from the terminal, radio network or the network.
Position calculation can be conducted, for example, by a positioning server (e.g. E-SMLC or SLP in LTE) or UE. The former approach corresponds to the user equipment-assisted positioning mode, whilst the latter corresponds to the user equipment-based positioning mode.
Two positioning protocols operating via the radio network exist in LTE, LPP and LPPa. The LPP is a point-to-point protocol between a LCS Server and a LCS target device, used in order to position the target device. LPP can be used both in the user and control plane, and multiple LPP procedures are allowed in series and/or in parallel thereby reducing latency. LPPa is a protocol between eNodeB and LCS Server specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. SUPL protocol is used as a transport for LPP in the user plane. LPP has also a possibility to convey LPP extension messages inside LPP messages, e.g. currently OMA LPP extensions are being specified (LPPa) to allow e.g. for operator-specific assistance data or assistance data that cannot be provided with LPP or to support other position reporting formats or new positioning methods.
A high-level architecture, as it is currently standardized in LTE, is illustrated in FIG. 1, where the LCS target is a terminal, and the LCS Server is an E-SMLC or an SLP. In the figure, the control plane positioning protocols with E-SMLC as the terminating point are shown in blue, and the user plane positioning protocol is shown in red. SLP may comprise two components, SPC and SLC, which may also reside in different nodes. In an example implementation, SPC has a proprietary interface with E-SMLC, and Llp interface with SLC, and the SLC part of SLP communicates with P-GW (PDN-Gateway) and External LCS Client.
Additional positioning architecture elements may also be deployed to further enhance performance of specific positioning methods. For example, deploying radio beacons is a cost-efficient solution which may significantly improve positioning performance indoors and also outdoors by allowing more accurate positioning, for example, with proximity location techniques.